Add contributing link to the nav

[rgaiacs]

Motivation

@ErinBecker query me and @mkuzak about if the contributing instructors was easy to find
and I think that it need more visibility on the website.

Screenshot

[mkuzak]

+1 I think at the top it's more visible, but we need to avoid catering the nav too

[rgaiacs]

but we need to avoid catering the nav too

For mobile devices we have the navigation bar as a list.

Screenshot of navigation bar toggle off:

Screenshot of navigation bar toggle on:

[mkuzak]

yes, so I guess we don't need to worry about cluttering :)